1. Field of the Invention
The field of art to which this invention relates is an apparatus for polarization conversion. It is more particularly directed to such apparatus which make use of reflective polarizer films and light sources having parabolic mirrors.
2. Discussion of the Prior Art
Intensity in optical projectors is usually limited by source brightness. In addition, projectors that use polarized light, such as many projection displays, suffer further intensity loss if the unused polarization is discarded. Frequently, an effort is made to capture the rejected component and re-inject it into the system after rotating its polarization to match that of the primary component. FIG. 1 illustrates a known arrangement for accomplishing this.
FIG. 1 shows a lamp 102 providing light having both S and P linear polarizations (where S polarization is denoted by "o" and P polarization is denoted by ".smallcircle."). The light enters a polarization beam splitter (PBS) cube 104 which transmits light of one of the polarizations, P polarization in this example, and reflects light of the other polarization, S polarization in this example. The reflected S polarized light is again reflected by a mirror 106 and directed towards a 1/2 waveplate 108, where its polarization is converted to P polarization. Thus, the incident light is converted to one polarization and directed to a lens 110 or other optical component.
Two problems with the FIG. 1 approach are added cost and the need to increase optical etendue (NA times field size) when one polarized source image is, by itself, large enough to fill the lens pupil. Increases in etendue add to cost, and the most cost effective step is usually to use a large enough lamp 1 that the image in a single polarization almost fills the pupil aperture chosen; this reduces the benefit from re-injecting the second polarization. Because of arc inhomogeneities, the FIG. 1 arrangement can, in practice, still provide some intensity increase, but the benefit is limited.
A known way to partially circumvent the etendue problem is to recycle the converted polarization through the arc, as shown in FIG. 2. It is thermodynamically impossible to increase the brightness of a black-body source of fixed temperature, but arc discharges are not fully opaque. The FIG. 2 system increases the effective source emissivity by redirecting rays through the arc. (Emissivity equals absorbance, according to Kirchoff's Law. By tracing rays backwards through the FIG. 2 system, one can see that the recycling elements also increase arc absorbance).
FIG. 2 shows a lamp 102 having a parabolic mirror 102a. The lamp provides both S and P polarized light, both of which pass through a 1/4 waveplate 114 and are directed to a PBS 104. The P polarized light is transmitted 116 and the S polarized light 118 is reflected to a mirror 112. The S polarized light is then reflected back to the PBS 104 and again reflected back towards the parabolic mirror 102a, first passing through the 1/4 waveplate where it is converted to circular polarized light having a right handedness 120. The circularly polarized light having a right handedness 120 is then reflected by the parabolic mirror 102a which converts its handedness to left-handedness 122. This light then reflects of the opposite side of the parabolic mirror 102a which converts its handedness back to right handedness 124. The circular polarized light having a right handedness 124 then passes back through the 1/4 waveplate 114 once again, which converts the light back to linear polarized light but having S polarization 126. The S polarized light 126 is then reflected once again by the PBS 104 towards the mirror 112, and back again towards the lamp 102. In this embodiment, an increase in brightness is not obtained unless there is a phase difference between the S and P components of the reflected light at the parabolic mirror 102a.
The return mirror 112 in the FIG. 2 system can be slightly tipped so that the two arc images are only partially overlapped; this can improve collected intensity when the system is not fully brightness-limited (due to arc inhomogeneities). However, in practice, the FIG. 2 arrangement is typically reported to have limited efficiency in converting the returned polarization to the desired output state. Also, in most projectors, the PBS 104 in the FIG. 2 system must be added as a new component (though in a few systems, a PBS 104 already present in the optics can also perform the recycling function). A PBS 104 is a fairly expensive optical component.
What is needed is a way to improve the efficiency of recycling and, at the same time, lower its cost.